1. Field of the Invention
The present invention relates generally to a method and apparatus for cleaning substrates in preparation for the deposition of thin film coatings, and more particularly to a method and apparatus for cleaning cathode ray tube (CRT) screens and other substrates such as those used in the flat panel display industry, all in preparation for the application of thin film coatings via physical or chemical vapor deposition processes.
2. Description of the Prior Art
A variety of techniques currently exist for depositing a thin film coating onto a substrate. Such techniques include electron beam deposition, chemical vapor deposition and sputtering, among others. Substrates to which thin film coatings are commonly applied include glass and to a lesser extent plastic, although the thin film coating of plastic, metal and ceramic substrates is also possible. Specific examples include glass and plastic substrates used in the flat panel display industry and CRT screens and panels used in the manufacture of computer monitors and television sets. Thin film coatings are applied to the above and other substrates for the purpose of providing such substrates with durability or with functional, electrical, optical, corrosion resistant or other properties.
Substrate cleaning and preparation is a necessary adjunct of physical and chemical vapor deposition processes for the production of thin film coatings. Such cleaning is necessary to remove contaminants from the surface of the substrate to be coated so that a stable coating can be applied. The existing art for substrate cleaning prior to thin film deposition commonly involves use of a liquid or combination liquid and vapor and generally takes one of two paths: aqueous and solvent. Economic, environmental and safety considerations usually dictate an aqueous cleanser as the cleaning agent of choice, especially for large scale industrial applications. Cleaning techniques using aqueous cleansers can include immersion, rinsing, mechanical scrubbing, application of acoustic energy, among others. In cases where a high level of cleanliness is required, the last stage can include a rinse with deionized water which can be relatively expensive.
Although aqueous cleaning of substrates is acceptable for many applications, it has several limitations. First, it is difficult to keep the final rinse water clean and free of all residue. This is a particular concern with substrates used in flat panel displays where micro deposits of residue on the substrate can cause nucleation defects in the coating. This in turn can lead to breaks in the patterns etched into the substrate. Second, it is difficult to dry the substrate in an efficient manner. This presents a problem because water vapor is incompatible with most vacuum coating processes. Total removal of water generally involves long outgassing periods before the coating process begins. Third, substrates which have complex shapes or assemblies are likely to trap water during the cleaning process, thus making it particularly difficult to remove all water from these areas. A specific example is a CRT which commonly includes a metal rim band surrounding the periphery of the face plate. The rim band is commonly applied over a band of fiber glass, rubber, plastic or paper which is positioned between the rim band and the glass envelope of the CRT. This structure provides numerous recesses, gaps and materials which may trap the aqueous cleaning medium. Removal of such medium from these spaces ordinarily requires prolonged exposure to a dry atmosphere, or heating or exposure to reduced pressure to volatilize the medium. The time and energy required to accomplish such removal often render the process uneconomical.
Solvent cleaning has also been used in some applications to prepare substrates for thin film coating. However, because solvents are also liquids, solvent cleaning involves many of the same limitations as aqueous cleaning. For example, it is difficult to keep the final solvent rinse totally free of residue and solvent can be still be trapped in areas where removal is difficult. Although solvents are generally more volatile than aqueous cleansers, and thus evaporate more quickly, it is this property that leads to environmental and safety concern since many of the solvents are petroleum based and thus emit volatile organic compounds (VOCs). Further, many of the nonaqueous cleansing solvents are toxic, flammable and expensive.
Accordingly, there is a need in the art for a method and apparatus for cleaning substrates in preparation for thin film coating in which such method and apparatus is economically feasible, is environmentally sound, is dry, facilitates the efficient removal of all residue and acceptably cleans the substrate.